In respiratory treatment, a patient is connected to a ventilator that controls and/or supports the patient's breathing. The ventilator typically includes a means of mixing and forming a breathing gas having a predetermined ratio of one or more gases, the pressurized sources of which are connected to the ventilator. The ventilator could also possibly include internal means to compress ambient air. The gas mixture from the ventilator must contain sufficient amounts of oxygen for the treatment of the patient. For this reason, one of the gases is always O.sub.2, or alternatively, in the extremely simplified case, the one single gas source is air. Other gases that are often mixed with O.sub.2 are typically air (N.sub.2 and O.sub.2), and sometimes also helium.
To perform the mixing function, each of the gas flow paths has a regulating means, typically a valve, to regulate the gas flow. In the current state of the art technology, these regulators are driven by microprocessor control units according to information received by the controller from various pressure, flow, temperature and/or position sensors. The microprocessor compares this received information from the various sensors to predetermined control parameters and drives the regulators via feedback control. Mechanical mixers using flow resistance ratios are also used in some simple applications to supply a single flow controlling valve.
The patient is connected to the ventilator through a breathing circuit that includes an inspiratory limb, an expiratory limb, and a patient limb. These three limbs of the breathing circuit are connected together at a Y-piece connector. Breathing circuits are generally classified as either an open circuit or as a rebreathing circuit. An open breathing circuit is most often used in intensive care applications, whereas a rebreathing circuit is typically found in anesthesia systems. In both types of breathing circuits, the inspiratory limb conducts the gas to be inspired from the ventilator to the Y-piece connector. The expiratory limb conducts the expired gas from the Y-piece connector back to the ventilator. In an open circuit, the expiratory limb is connected to an expiratory valve that functions to regulate the expiratory pressure and discharges the gases to atmosphere. In a rebreathing circuit, the expiratory limb is connected to the ventilator for CO.sub.2 removal.
In each type of breathing circuit, the patient limb connects the patient to the Y-piece connector and conducts inspired gases into the lungs and the expired gases back to the Y-piece connector. The patient limb typically includes an endotracheal tube, which is inserted into the trachea through the nose or mouth of the patient. Functionally, other equipment such as sensors, heat and moisture exchangers, and sampling connectors can be positioned between the Y-piece connector and the endotracheal tube.
When the patient is inspiring, the expiratory limb is closed through valves or other flow directing means depending on the breathing circuit. During inspiration, the ventilator forces the inspired gas to fill the patient's airways and lungs through the inspiratory and patient limbs with overpressure. During patient expiration, which is driven by the passive recoil of the lungs caused by the lungs' elastic force, the breathing circuit is arranged to conduct the exhaled gas through the patient and expiratory limbs back to the ventilator.
Modern ventilators used in intensive care applications use a base flow, which is a flow of gas supplied by the inspiratory valve to the inspiratory limb during the expiratory phase of the breathing cycle. The rationale behind the use of a base flow is to make it possible for the ventilator to detect an inspiratory effort by the patient with a minimal addition of extra work for the patient. In ordinary anesthetic rebreathing systems, fresh gas is delivered into the breathing system continuously from a fresh gas inlet.
The volume of breathing gas injected into the lungs during the inspiratory phase, also called the tidal volume (V.sub.t), ranges from a few tens of milliliters for a newborn child up to more than one liter for a large adult. The rate at which the tidal volumes are delivered, called the respiration rate, varies from a few tens of breaths/min. down to a few breaths/min. depending on the patient size. The typical range of breathing gas volumes per minute varies according to body weight and condition, but typically ranges from less than 1 liter/min. up to more than 10 liters/min.
The inspiratory tidal volume (V.sub.t) is normally delivered during the first third of the breathing cycle. Thus, the peak inspiratory flow may easily exceed thirty liters/min. and may momentarily reach more than 100 liters/min. in inspiration based on the control of a pre-set inspiratory pressure.
The tidal volume (V.sub.t) that ventilates the patient's alveoli can be divided into two parts, an alveolar volume (V.sub.A) and a deadspace volume (V.sub.D). The alveolar volume (V.sub.A) is defined as the volume of fresh gas that reaches the gas exchanging part of the patient's lung. In the alveoli, the high partial pressure of oxygen compared to the pressure of the perfused pulmonary blood flow makes the oxygen diffuse through the alveolar membrane and into the blood, such that the blood can transport the oxygen to tissues having a demand for oxygen. In an opposite manner, the high partial pressure of carbon dioxide (CO.sub.2) in the perfused pulmonary blood flow, as compared to the low pressure of CO.sub.2 in the fresh gas, makes the CO.sub.2 diffuse through the alveolar membrane into the alveoli to be washed out in expiration.
The deadspace volume (V.sub.D) is defined as the volume of fresh gas remaining in the patient limb and the upper airways of the patient, i.e., the trachea and bronchi at the end of inspiration. The deadspace volume does not take part in the gas exchange since it never reaches the patient's lungs.
Typically, the deadspace gas volume of a healthy human is in the order of 30% of the resting tidal volume, such as approximately 150 ml of deadspace in a 70 kg adult. In a patient with lung disease, the deadspace volume (V.sub.D) can be significantly increased.
The composition of the tidal volume (V.sub.t) is clarified in FIG. 1, where FIG. 1A represents the tidal volume (V.sub.t) divided into the alveolar volume (V.sub.A) 1 and the deadspace volume (V.sub.D) 2. Thus, the tidal volume is a simple sum of the alveolar 1 volume (V.sub.A) 1 and the deadspace volume (V.sub.D) 2. FIG. 1b diagrammatically represents the gas exchange situation within the lungs at the beginning of inspiration by the patient, after the exhalation flow has stopped. At this point in time, the upper airways and the patient limb forming a deadspace 4 are filled with exhaled gas 3. As shown in FIG. 1b, the gas volume remaining within the patient's lungs is represented by the reference numeral 5.
FIG. 1c shows the situation at the end of the inspiration by the patient. The volume of exhaled gas 3 previously present in the deadspace 4 has been sucked back into the patient's lungs 5, followed by the alveolar volume 1 of fresh inspiratory gas. The combination of the alveolar volume 1 and the exhaled gas 3 have increased the volume of the lungs by the amount designated by reference numeral 6. The deadspace comprised of the patient limb and the upper airways is now filled with fresh gas, referred to as the deadspace volume 2, which does not participate in the gas exchange within the lungs. Thus, the only volume participating in the gas exchange is the alveolar volume 1. Since the increase in lung volume is equal to the tidal volume (V.sub.t), the volume of exhaled deadspace gas 3 is equal to the deadspace volume (V.sub.D) 2.
The human body attempts to maintain a constant CO.sub.2 partial pressure, P.sub.ACO2, in the blood. Thus, a requirement exists for a certain amount of alveolar ventilation, see equation 1, which is determined by the patient's need for the CO.sub.2 elimination necessary to maintain the constant CO.sub.2 partial pressure, as set out in equation 2. The elimination of CO.sub.2 cannot be significantly affected by increasing the concentration gradient, since the inspiratory CO.sub.2 fraction is already close to zero. On the contrary, with the supply of more oxygen to the patient, the inspiratory oxygen fraction can be increased, which effectively increases the O.sub.2 concentration gradient, the O.sub.2 partial pressure, and thus decreases the CO.sub.2 partial pressure. ##EQU1##
whereas: ##EQU2##
The pressure buildup in the lungs during ventilator assisted inspiration depends on lung mechanics characteristics and the delivered volume, as shown in equation 3. A high peak or average lung pressure affects the pulmonary blood perfusion in a negative way and may reduce the net amount of oxygen transported from the lungs to the tissue. Also, a high lung pressure may cause a rupture in the lung tissue, specifically when the tidal volume exceeds the lung elastic variation limits. Therefore, it is desirable to keep the alveolar pressure low so as to reduce the negative effect on the pulmonary blood flow and avoid possible lung damage. However, a certain pressure level is necessary to inject the needed tidal volume (V.sub.t)into the lungs. ##EQU3##
whereas: C=Lung compliance
Based on the above discussion, it can be seen that the effectiveness of gas exchange, specifically the removal of CO.sub.2, is defined by the alveolar volume (V.sub.A) (equations 1 and 2), while the detrimental effects of ventilation are related to the increased pressure created by an increased tidal volume (V.sub.t). The alveolar ventilation is determined by the gas exchange requirement and cannot be eliminated beyond a minimum without special and extremely expensive extracorporeal membrane oxygenation and carbon dioxide removal treatment. An alternative for improved effectiveness of the ventilation system is the reduction or preferably elimination of the deadspace volume (V.sub.D), since the deadspace volume provides no gas exchange benefits to the patient.
Tracheal gas insufflation, abbreviated as TGI, is a method for the reduction of the deadspace volume (V.sub.D) of exhaled gases. In the TGI method, the patient limb is flushed with fresh gas to reduce the amount of deadspace volume of exhaled gases. For this purpose, a special double lumen endotracheal tube is used to deliver the flushing gas flow at the distal end of the patient limb tube, thereby filling the patient limb tube with oxygen enriched gas having little to no carbon dioxide. During the next inspiration by the patient, the volume of exhaled gas pushed back into the lungs is greatly reduced. The tidal volume required to provide a desired alveolar volume (V.sub.A) can be decreased correspondingly.
In many instances, the volume of the patient limb and upper airways can be as high as 100 ml. Typically, the tidal volume (V.sub.t) is approximately 500 ml, so that the deadspace gas volume (V.sub.D) represents approximately 20% of the tidal volume. By reducing the volume of exhaled gases in the deadspace volume, the maximum pressure set forth by equation 4 can be reduced, thereby reducing the negative effects caused by high lung pressure as described above.
As discussed above, reduction of the exhaled gases in the deadspace volume by expiratory flushing of the airway improves CO.sub.2 elimination. A considerable body of experimental work in animals indicates the clinical potential of TGI in an intensive care setting. Although deadspace flushing is unlikely to be the only operative mechanism, the efficiency of TGI increases when the contribution of the TGI volume is proximal to the orifice of the flushing catheter to the total physiologic deadspace.
An important advantage in the reduction of the deadspace exhaled gas volume is the ability to reduce lung pressure while maintaining the required CO.sub.2 elimination. When feeding TGI gas into the airway of the patient, some of the added gas will always be mixed with the patient gas distally of the TGI outlet, thus providing an efficient flush volume that can be greater than the actual volume proximal to the TGI outlet.
A comparison of continuously feeding a constant flow of oxygen and a pulsatile flow during the expiratory phase only into an outlet located in the distal end of the endotracheal tube was made in an article written by J. Marini entitled "Tracheal Gas Insufflation as an Adjunct to Ventilation". Although a continuous flow of oxygen gives a good deadspace wash, it has the drawback in that it adds to the maximum lung pressure unless compensated with ventilator adjustments. The pressure increases since the TGI flow is added to the inspiratory flow increasing the tidal volume with the previously mentioned detrimental effects. Additionally, since the device of the Marini article adds oxygen to the inspiratory flow, it will effect the actual oxygen fraction delivered to the patient. The result of the deadspace volume being filled with pure oxygen during expiration also adds to the uncertainty of the actual oxygen fraction delivered to the patient. Limiting the TGI flow selectively for the expiratory phase avoids the disadvantage of end-inspiratory overdistension.
A method for the delivery of the TGI is presented in WO 97/31670. In this reference, equipment is operated during the expiratory phase only. A further improvement on the ideas presented by Marini is the possibility of adjusting both the TGI flow rate and the TGI gas composition. This improvement is made possible by adding a dedicated mixer of oxygen and air with an adjustable needle valve for mixed gas flow rate adjustment to supply the gas for synchronizing TGI delivery apparatus. This apparatus receives the expiratory synchronization signal from the ventilator through a special signal line. The disadvantage of this system is the complexity and bulkiness for an already crowded intensive care environment. The device requires dedicated high pressure gas connection lines and mixers, and a separate adjustment for the gas mixture of the TGI to match the ventilator mixture. Additionally, the device shown in the above reference has to be customized with the make of the ventilator for the synchronization signal. One further problem of the described system arises from a patient safety standpoint. Since the system has its own high pressure supply connections, a risk of unintentional gas dosing exists, which results in overdistension of the patient's lungs. To eliminate this problem, separate pressure sensing and supervisory equipment are included in the described TGI dosing equipment.